Acid-Base Site Detection and Mapping on Solid Surfaces by Kelvin Force Microscopy (KFM)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Electrostatic potential at the surface of acidic or basic solids changes under higher relative humidity (RH), as determined by using Kelvin force microscopy (KFM). The potential on acid surfaces becomes more negative as the water vapor pressure increases, while it becomes more positive on basic solids. These results verify the following hypothesis: OH- or H+ ions associated with atmospheric water ion clusters are selectively adsorbed on solid surfaces, depending on the respective Bronsted acid or base character. Therefore, Kelvin microscopy, under variable humidity, is a rigorous but convenient alternative to determine the acid-base character of solid surfaces, with a great advantage: it uses only one amphoteric and simple reagent to determine both the acid and base sites. Moreover, this technique provides information on the spatial distribution of acid-base sites, which is currently inaccessible to any other method.84231019110198Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Water with Excess Electric Charge

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Water dropping from a biased metal needle acquires net electric charge that can be stored within. a Faraday cup. Positive (negative) water is obtained from a positive (negative) needle, and its charge largely exceeds the Rayleigh limit. Water drops transform into water threads at V < -9.5 kV, even under field strengths much lower than those used in electrospray, electrowetting, or electrospinning experiments. Surface tension of electrified water decreases as the positive or negative charge increases, but its density and viscosity are the same as those of neutral water. Water drops falling in the air through a biased noncontacting metal ring also acquire charge but with opposite sign to the metal potential, in agreement with a recent model for the electrification of insulators or isolated metals, based on charge transfer coupled to water evaporation, adsorption, and desorption.115221122611232Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP